Simple readers for cards, magnetically encoded along a longitudinal strip, conventionally having a motor-driven driving wheel which engages the surface of the card for delivering it between a reader-writer head and a pressure anvil which may be presented directly opposite the reader-writer head. In conventional equipment heretofore in use, either the anvil or the head has been held by a spring device. Vibrations, as between the head and the anvil, have caused errors in reading and writing, and may contribute to severe head wear, requiring frequent replacement.
When a card to be read starts to pass between the head and the pressure anvil, in a conventional system the physical movement perpendicular to the plane of the card and equal to its thickness, may initiate vibrations which are not readily damped. A less obvious but more continuing source of such vibrations appears to be intermittency of friction in the drive system, accompanying longitudinal movement between the read-write head and its anvil. These sources, as well as still other causes, may excite vibrations in the head or its anvil; if the length of a bit of magnetically encoded information is short relative to the period of vibration, there may be serious malfunctions in the read-write function. Where accurate reading and writing are imperative, as with "debit" cards wherein encoded information is rewritten in modified form upon the performance of a command, such vibrations may seriously impair the functioning of the system.
As contrasted with magnetically encoded cards used for simple functions, for example, to open the gate of a parking lot, a magnetically encoded debit card must be reliably resistant to errors produced by vibrations and intermittency in magnetic reading and writing. Spurious variations in pulse time characteristics, termed "jitter," may follow from such vibrations.